An adhesive agent containing a thermoplastic block copolymer as a main component has been used in disposable products typified by a paper diaper and a napkin and, particularly, a hot-melt adhesive agent based on a styrene class block copolymer has widely been used. For example, a paper diaper is produced by bonding a polyethylene film with other members (for example, a nonwoven fabric, an elastic material such as a natural rubber, a water-absorbing paper, etc.) using a hot-melt adhesive agent. The hot-melt adhesive agent can be applied to various members using various methods and, even when using any method, the hot-melt adhesive agent is melted by heating so as to obtain an appropriate viscosity, and then the molten adhesive agent is applied to various constituent members in a dot, linear, stripe, spiral or sheet form.
It is now required for the paper diaper to improve drapeness thereof, and a study has been made in improving flexibility and drapeness of the paper diaper by more thinning a polyethylene film or the above-mentioned various members such as a nonwoven fabric. Thinning of various members significantly reduces material costs. However, thinning of the polyethylene film may cause a problem that heat resistance deteriorates and application of a high-temperature (not lower than 150° C.) hot-melt adhesive agent leads to melting of the polyethylene film or formation of wrinkles of the polyethylene film. Therefore, adhesive agent manufacturers have made a progress on the development of a low-temperature-applicable hot-melt adhesive agent which is capable of applying at low temperature (not higher than 140° C.).
Taking workability and environmental aspect in the case of application of the hot-melt adhesive agent into account, manufacturers producing a paper diaper and a sanitary good strongly desire lowering of the viscosity of the hot-melt adhesive agent. The hot-melt adhesive agent commonly comprises a base polymer and a plasticizer, and a study has been made in lowering the viscosity of the hot-melt adhesive agent by a method in which the amount of the base polymer is decreased to thereby increase the amount of the plasticizer. However, the production of a paper diaper using a low viscosity hot-melt adhesive agent produced using such method may cause a problem that the balance between an adhesiveness to a polyethylene film which composes members of the paper diaper and a retention force (cohesive force) is deteriorated, and the softening point is excessively lowered.
There is also a paper diaper including a rubber thread incorporated thereinto. In the case of incorporating the rubber thread into the paper diaper, the drawn rubber thread is bonded to a paper diaper unit. A hot-melt adhesive agent is usually used as an adhesive agent. The paper diaper unit commonly has no elasticity. Therefore, the paper diaper including the rubber thread bonded thereto is folded by a shrinkage force of the rubber thread when the rubber thread bonded to the paper diaper shrinks. As a result, expansion and shrinkage forces of the rubber thread are applied to the paper diaper unit, and thus enabling the paper diaper to fit the body.
However, if the hot-melt adhesive agent used in bonding of the rubber thread has insufficient creep resistance, it becomes impossible for the hot-melt adhesive agent to hold the rubber thread, which is inclined to shrink, at the position where the rubber thread was bonded to the paper diaper unit. That is, only the rubber thread shrinks without accompanying the paper diaper unit. In that case, even if the rubber thread shrinks, paper diaper unit is not folded and thus expansion and shrinkage forces of the rubber thread are not applied to the paper diaper unit. Therefore, the paper diaper fails to fit the body. In order to solve such inconvenience, the hot-melt adhesive agent is required to have excellent creep resistance together with excellent adhesiveness.
Patent Literatures 1 to 4 disclose hot-melt adhesive agents based on a styrene class block copolymer.
Patent Literature 1 and Patent Literature 2 mention hot-melt adhesive agents including a radial type styrene block copolymer (claim 1 of each patent literature). However, the hot-melt adhesive agents of these patent literatures are unsuitable for application at low temperature since they include a high-viscosity radial type styrene block copolymer, and have neither sufficient tack nor adhesiveness because of a low diblock content of the radial type styrene block copolymer.
Patent Literature 3 discloses a hot-melt adhesive agent including a styrene-butadiene-styrene block copolymer having high styrene content. The hot-melt adhesive agent of Patent Literature 3 is insufficient in any one of application at low temperature, creep resistance and adhesiveness to a polyolefin substrate, as shown in paragraph [0068] of Table 1 and paragraph [0072] of Table 2. Therefore, taking producibility of a paper diaper into account, it is impossible to say that the hot-melt adhesive agent of Patent Literature 3 completely meets high requirements of product manufacturers.
Patent Literature 4 discloses a hot-melt adhesive agent including an ultra-branched styrene type block copolymer (see [claims]). As mentioned in paragraphs [0048] and [0085] to [0088], a viscosity of the adhesive agent including an ultra-branched styrene type block copolymer (Soloprene 9618) is measured at 163° C. Therefore, the hot-melt adhesive agent of Patent Literature 4 does not also take application at low temperature of not higher than 140° C. into account. Reduction in the content of the ultra-branched type styrene type block copolymer enables application of a hot-melt adhesive agent at low temperature, but causes deterioration of the adhesiveness to the polyolefin substrate.
Patent Literature 1: JP 5-311138 A
Patent Literature 2: JP 2006-8947 A
Patent Literature 3: JP 2010-506005 W
Patent Literature 4: JP 2010-536957 W